1. Field of the Invention
The present invention relates to transceivers and, in particular, to detecting a cable disconnect to provide a hot connection capability.
2. Description of the Related Art
Transceivers (combination transmitter/receiver devices) are widely utilized for digital and analog communication of electrical signals. Transceivers are often connected to each other by external cables. In some transceiver systems, the cable may be connected and disconnected while the system is running. This is sometimes referred to as a xe2x80x9chot connection.xe2x80x9d To implement a transceiver system with a hot connection capability, the transmitting transceiver must be able to detect if the cable is disconnected within a relatively short time.
One type of transceiver that is utilized and that can be configured to allow hot connections to be made is a USB transceiver, i.e. a transceiver designed in accordance with the Universal Serial Bus (USB) specification, e.g. USB Specification, rev. 1.1 (Apr. 20, 1998). USB is well-known to those skilled in the art; a technical specification on the bus can be found on the World Wide Web at  less than http://www.ti.com/sc/docs/msp/usb/spec/spec1.htm greater than . Additional information on USB may be found at  less than http://www.intel.com/design/usb/ greater than ;  less than http://www.usb.org greater than ;  less than http://www.usb.org/developers/index.html greater than . The two xe2x80x9csidesxe2x80x9d of a communication using transceivers may sometimes be referred to as the xe2x80x9chostxe2x80x9d side and the xe2x80x9cdevicexe2x80x9d side of the system or communication.
Referring now to FIG. 1, there is shown a high-level block diagram of a prior art communication system 100 having two transceivers X1, X2, connected by a cable 101. Transceiver X1, consists of transmitter T1 and receiver R1. Transmitter T1 may be put into a high impedance state by means of enable signal EN1 when X1 is used in the receive mode. Transceiver X2 is similar to X1 and consists of transmitter T2 and receiver R2. Cable 101, consisting of one or more wires, connects X1 and X2.
Differential transceiver connectivity may also be employed. Referring now to FIG. 2, there is shown a block diagram of a prior art transceiver communication system 200 with differential transceiver connectivity. Differential transceiver system 200 has two special, differential transceivers X1, X2, connected by a cable 201. Transceiver X consists of1 differential transmitter T1 and differential receiver R1. Transceiver X2 is configured similarly.
Referring now to FIGS. 3A,B, there are shown circuit diagrams illustrating transceiver T1 and receiver R2 of transceiver system 200 of FIG. 2 in further detail. (Transceiver T2 and receiver R1 of system 200 are configured similarly.) Transmitter T1 steers a current I1 through a termination resistor RES2 of receiver R2, whose value matches the impedance ZO of the cable connecting these two components (e.g., a cable such as cable 201). Typical values of ZO vary from 20xcexa9 to 150xcexa9, depending on the type of cable employed. Receiver R2 is merely a differential comparator. When input A is low and its inverse AN is high, transistors M1 and M4 are on, and transistors M2 and M3 are off. If switch SW2 coupled to resistor RES2of receiving transceiver X2 is closed, the current I1 is steered from node N, through resistor RES2, back into node P, and then to ground. Thus, node P is at a lower voltage than node N, so the output OUT2 of the receiver R2 is low. When A is high and AN is low, the current I1 is steered in the opposite directions, so that node N is at a lower voltage than node P, so the receiver""s output is high.
Referring once more to FIG. 2, the termination resistors RES1, RES2 may be placed at both sides of cable 201, or at only one side. In order to minimize reflections from impedance mismatch, it is preferable to have a termination resistor on the side that is receiving. Placing another termination resistor on the side that is transmitting may help signal integrity further, but it comes at the price of doubling the power that is needed to produce and transmit a given signal.
This may be seen in the following example. Assume a 50xcexa9 cable impedance and a single 50xcexa9 termination resistor, placed at the receiver side. Typical differential signals are 200 mV to 800 mV. If a 400 mV signal is desired, this is achieved by having a current of 8mA flow through the 50xcexa9 termination resistor. With a 3.3V power supply, this means a DC power consumption of 26.4 mW. However, if there is a 50xcexa9 termination resistor at each side of the cable, a 16 mA current is needed to yield the same 400 mV signal. This results in a DC power consumption of 52.8 mW. For this reason, it is often a good idea to have the switch (SW1 or SW2) open at the side that is transmitting and closed at the side that is receiving.
In order to detect cable disconnect, present USB transceivers use two external 15 Kxcexa9 pull-down resistors at the host side of the cable, and a 1.5 Kxcexa9 pull-up resistor at the device side of the cable. As long as the cable is connected, the host senses that one connection of the (differential) gain signal is high and the other is low. If the cable is disconnected, however, both connections are pulled low by the 15 Kxcexa9 pull-down resistors. In this manner, the host side may detect whenever the cable has been disconnected.
This disconnect-detection technique has several disadvantages. First, it only works for the host side of the cable, not the device side. Second, this technique is relatively slow, and may be unacceptably slow for some applications. The RC time constant is typically 4.5 xcexcs, since an approximately 300 pF load must be pulled down through the 15 Kxcexa9 pull-down resistor. (Lower value resistors could be used to reduce the time constant and to speed up the detection, but this would utilize more DC power, which is also undesirable.) Third, this technique requires the use and presence of the external pull-down resistors at the host side.
A transceiver comprises a transmitter for transmitting a transmitted signal to a second transceiver via a cable; a first receiver for receiving a received signal via the cable from the receiving device; and a second receiver for detecting a disconnect condition of the cable when the transmitter is transmitting the transmitted signal.